A typical modern-day cardiac control device maintains the capability to sense cardiac electrical activity as well as a capability to deliver an electrical or pharmaceutical therapy. Although the present invention has application to any cardiac control device sensing channel in which noise is to be rejected, the invention can be understood most readily by considering a simple ventricular demand VVI pacemaker (ventricular pacing and sensing; inhibited mode). Typically, demand VVI pacemakers generate a stimulation pulse after a preset time interval in the absence of the sensing of a heart's spontaneous beat within a predetermined interval. If the pacemaker senses a spontaneous beat, it does not generate a stimulation pulse. Following either a stimulated (paced) or spontaneous heartbeat, the pacemaker presets the cycle interval timer and enters a new sensing interval, usually following an initial time period during which pacing is disabled. The pacemaker continuously repeats the cardiac cycle of operations. The demand VVI pacemaker employs electrodes which are implanted within the right ventricle of the heart both for delivering the stimulus and for sensing spontaneous heartbeats.
Noise in the sensing channel can lead to erroneous operation. In the worst case, the pacemaker may interpret continuous noise as representing rhythmic heartbeats and fail to generate pacing stimuli even when the heart is not beating properly. To prevent this occurrence, pacemakers are often designed to automatically convert from a VVI demand mode to a VOO fixed rate (ventricular pacing at fixed rate; no sensing) mode of operation upon the detection of noise which prevents the reliable sensing of spontaneous heartbeats. The automatic mode switch, which may be termed "noise reversion pacing", is disadvantageous because it prevents the heart from beating at a natural rhythm and increases power drain on the battery due to the possible generation of continuous, unnecessary stimulation pulses.
A pacemaker detects noise by defining a refractory sensing interval, during which sensed electrical events are classified as noise. Pacemakers are designed to provide absolute and relative refractory intervals. The absolute refractory period (ARP) is a fixed interval which immediately follows a stimulated or spontaneous heartbeat. During the ARP, sensing is totally disabled to allow the afterpotential from a stimulus and the heart's evoked response to dissipate. The relative refractory period (RRP), which immediately follows the ARP, is provided to allow detection of noise. During the RRP, the after-potential from the stimulus has dissipated and cardiac signals are quiescent, therefore noise detection is possible. Cardiac events should not occur during the RRP because it occurs too soon after a heartbeat. Therefore, the pacemaker classifies sensed events during the RRP as noise and restarts the RRP timer. As long as the RRP is in progress, sensed events verify that noise is present, causing the pacemaker to restart the RRP timer and, therefore, delaying sensing for the purpose of inhibiting stimulation pulse generation. If the pacemaker continually restarts the RRP through the time that a pacing stimulus is due, it will switch to noise reversion pacing and generate the pacing stimulus, as stated previously.
Other types of cardiac control devices, which are not pacemakers, sense noise in a different manner but may respond in a similar manner. For example, antitachycardia pacemakers and defibrillators may define a noise window at some interval following a pacing stimulus or spontaneous heartbeat (e.g., about 50 to 100 msec post-event). When the device senses events in the noise window in a particular preponderance of cardiac cycles (e.g., 15 of 16, or 7 of 10), it will initiate VOO pacing. Inherent in this response to noise sensing are the drawbacks of noise reversion pacing (i.e., preventing the heart from beating at a natural rhythm and elevated power drain). In addition, these sensing methods may fail to distinguish noise from fibrillation. A VOO pacing response to fibrillation is woefully inadequate. Since antitachycardia pacemakers and defibrillators are intended for usage in heart patients who are at risk of arrhythmia episodes, it is critical that such a device can distinguish fibrillation from noise.
Noise rejection systems in the prior art have generally involved the making of adjustments to the pacemaker sensitivity. (For example, see U.S. Pat. No. 4,516,579, invented by Irnich and entitled "Interference Recognition Circuit in a Heart Pacemaker", issued May 14, 1985.) Sensitivity refers to the magnitude of an input signal which is sufficient to cause the pacemaker to sense a cardiac event. The act of lowering the sensitivity reduces the effect of noise by requiring a higher input signal level to exceed the threshold. These noise rejection systems are effective only in response to continuous noise. Whigham, in U.S. Pat. No. 4,779,617, entitled "Pacemaker Noise Rejection System", issued Oct. 25, 1988, discloses a noise rejection system which is designed to reduce electrophysiological noise, such as noise arising from a patient's skeletal muscles. This noise rejection system also operates by adjusting the pacemaker's sensitivity setting.
Other prior art devices reduce the influence of noise on a pacemaker system by filtering the data stream that enters into the sensing circuit, rather than adjusting sensitivity. Belt, in U.S. Pat. No. 4,436,093, entitled "Cardiac Pacer Having Active Notch Filter System", issued Mar. 13, 1984, discloses a noise rejection system which filters the electrical signal sensed within the heart to reduce continuous line frequency noise. However, this system does not improve noise immunity to skeletal muscle noise or bursts of line frequency noise.
Other filtering techniques, including fixed and adaptive digital methods, have been implemented to reduce the influence of line frequency noise. The overriding disadvantage of these methods, at least for applications in current implantable devices, is circuit or software complexity and a requirement for floating point computations.
Other less computationally-demanding methods involve morphology analysis. One example is an improved QRS detection method in which a device senses the amplitude of an electrical signal of the heart and counts the number of times the signal crosses a predetermined threshold value. A true sense occurs if the count number is within a preset range over a time interval of a particular duration. This method accurately detects QRS signals only if the threshold level is correctly set. Unfortunately, setting the threshold correctly is difficult due to large variations in the amplitude of QRS signals from patient to patient. Also, the accuracy of morphology detection techniques is limited due to potentially large beat-to-beat variability inherent in physiological systems.
A further problem afflicting present-day devices relates to the rejection of amplitude modulated or burst electromagnetic fields. One source of burst line frequency noise is faulty, or poorly designed, appliances where the patient is in contact with a line frequency AC powered device. The patient actually is part of an electrical pathway to ground. In contrast to sense detection in the presence of continuous additive line frequency interference, the operation of the sensing circuit during amplitude modulated or burst electromagnetic interference (EMI) is probably more important to patient safety. Burst line frequency noise is a potentially dangerous situation for pacemaker-dependent patients because burst noise may inhibit stimulus generation in a cardiac control device. The potential hazard of continuous line frequency noise, in comparison to burst noise, is less precarious because continuous line noise will cause the device to pace asynchronously with respect to a spontaneous cardiac rate, but the device will still support the patient.
All of the aforementioned noise rejection methods are intended to distinguish spontaneous cardiac events from noise arising from various sources. A supplementary objective, in the cardiac control system of the present invention, is noise reduction to permit a detailed analysis of electrophysiological signal waveforms.
A large physiological signal component exists at frequencies corresponding to worldwide line frequencies. For example, the frequency spectrum of intrinsic cardiac electrograms show significant power at frequencies right at, and around, 50 and 60 Hz. The general purpose highpass or lowpass filters of present-day cardiac control and monitoring devices are unable to filter out line noise without markedly attenuating physiologic signals. Hardware notch reject filters with sufficient Q to filter line frequencies but retain physiological signal components are inappropriate for an implantable system due to the requirement for additional circuit components and added current drain. Also, an integrated circuit capable of meeting the Q requirements would involve very difficult design and production problems arising from tight tolerances on the IC process to provide a filter notch at the specified frequency. Therefore, there does not exist an appropriate analog hardware filter solution to this problem for implantable applications.
Accordingly, it is the primary object of this invention to provide an improved physiological event signal sensing system for a cardiac medical device.
It is a further object of this invention to provide an improved physiological event signal sensing system that reliably senses natural cardiac events which are obscured by the presence of either continuous noise or pulsed noise in the signal sensing system.
Another object of the invention is a physiological event signal sensing system that provides improved detection of spontaneous cardiac events in the presence of bursts of power line frequency noise which occur either during or outside of the heart's refractory period.
A still further object of the invention is a physiological event signal sensing system that provides improved detection of natural cardiac events in the presence of continuous power line frequency noise, thus maintaining hemodynamic efficiency.
Yet another object of the invention is a physiological event signal sensing system that utilizes digital notch filtering and morphology analysis in providing improved detection of spontaneous cardiac events in the presence of bursts of power line frequency noise.
A further object of the invention is a physiological event signal sensing system that provides improved detection of natural cardiac events in the presence of noise in the signal sensing system, thus preventing asynchronous pacing (VOO or AOO mode pacing) when the heart is beating with a natural rhythm and conserving energy in an implantable system.
It is still another object of this invention to provide an improved physiological event signal sensing system that detects and logs information concerning steady state noise levels, and the occurrences of onset and offset of noise interference.
Further objects, features and advantages of this invention will become apparent as the following description proceeds.